Is it possible to increase TCP listen backlog for sshd?

Environment

• Red Hat Enterprise Linux 6, 7, 8, 9
• openssh-server

Issue

• Is it possible to increase TCP listen backlog for sshd?

Resolution

• No, the backlog argument of listen() is a hardcoded value with a macro definition, SSH_LISTEN_BACKLOG.

openssh-8.7p1-10.el9_0(RHEL9)/sshd.c:

static void
listen_on_addrs(struct listenaddr *la)
{
...
                /* Start listening on the port. */
                if (listen(listen_sock, SSH_LISTEN_BACKLOG) == -1)
                        fatal("listen on [%s]:%s: %.100s",
                            ntop, strport, strerror(errno));
                logit("Server listening on %s port %s%s%s.",
                    ntop, strport,
                    la->rdomain == NULL ? "" : " rdomain ",
                    la->rdomain == NULL ? "" : la->rdomain);
        }
}

openssh-8.7p1-10.el9_0(RHEL9)/ssh.h:

/* Listen backlog for sshd, ssh-agent and forwarding sockets */
#define SSH_LISTEN_BACKLOG              128

Diagnostic Steps

• Refer to How do I view the listen backlog of a TCP socket?.

# ss -npl src :22
Netid      State       Recv-Q      Send-Q           Local Address:Port            Peer Address:Port      Process                             
tcp        LISTEN      0           128                    0.0.0.0:22                   0.0.0.0:*          users:(("sshd",pid=722,fd=3))      
tcp        LISTEN      0           128                       [::]:22                      [::]:*          users:(("sshd",pid=722,fd=4))      
# sysctl net.core.somaxconn
net.core.somaxconn = 4096

This solution is part of Red Hat’s fast-track publication program, providing a huge library of solutions that Red Hat engineers have created while supporting our customers. To give you the knowledge you need the instant it becomes available, these articles may be presented in a raw and unedited form.